Turbidity sensor based on ultrasound measurements

ABSTRACT

A turbidity measurement device for measuring turbidity of a fluid flowing in a flow tube. A first transducer transmits ultrasonic signals through the fluid in the turbidity measurement section so as to provide a first ultrasonic standing wave between the first and second section ends. A receiver transducer receives the ultrasonic scattered response from particles in the fluid flowing through the turbidity measurement section. A control circuit operates the transducers and generates a signal indicative of the turbidity of the fluid in response to signals received from the receiver transducer. Preferably, the device may comprise a second transducer for generating a second ultrasonic standing wave with the same frequency, and further the two transducers may be used to generate a measure of flow rate by means of known ultrasonic techniques. This flow rate may be used in the calculation of a measure of turbidity. Both turbidity facilities and flow rate facilities may be integrated in a consumption meter, such as a heat meter or a water meter.

This non-provisional application is a continuation of U.S. patentapplication Ser. No. 15/741,560 filed on Jan. 3, 2018, which is anational phase of International Application No. PCT/DK2016/050235 filedJul. 1, 2016 and published in the English language, which is anInternational Application of and claims benefit of priority to EuropeanPatent Application No. 15175269.8, filed on Jul. 3, 2015; 15175270.6,filed on Jul. 3, 2015 and 15175271.4, filed on Jul. 3, 2015. Thedisclosures of the above-referenced applications are hereby expresslyincorporated by reference in their entireties.

FIELD OF THE INVENTION

The present invention relates to the field of turbidity sensors formeasurement of turbidity of a fluid. Especially, the invention providesa device capable of measuring turbidity based on ultrasonicmeasurements. Further, the invention provides an ultrasonic consumptionmeter, or utility meter, comprising such turbidity sensor.

BACKGROUND OF THE INVENTION

Worldwide the consumption of clean water for drinking is increasing.Drinking water is retrieved from underground wells, but also surfacewater or even de-salted sea water are used as drinking water. Thus,there is a demand for utility companies to measure cleanness of thewater supplied to the utility network.

A complete analysis of cleanness of water involves complicatedbiochemical analyses, however in some cases a measure of water qualityobtained by means of a turbidity measurement can be sufficient, i.e.measurement of the amount of particles in the fluid as a measure ofcleanness of the fluid. Such turbidity measurements can be based onoptical methods.

Optical turbidity equipment, however, is not well-suited for functioningas a permanently mounted part of a utility network due to the forming ofcoatings of minerals and/or biofilms on the optical surfaces, which willdisturb the turbidity measurements and necessitate frequent maintenance.Furthermore, such optical turbidity measurement equipment is expensiveand can thus in practice only be installed at a limited number ofpositions in a utility network.

SUMMARY OF THE INVENTION

It would be advantageous to provide a simple and low cost turbiditymeasurement device which is still robust and reliable for measuringturbidity of the fluid in a utility network, hereby allowing utilitycompanies to distribute such devices at several positions in the utilitynetwork.

In a first aspect, the invention provides a device arranged to measureturbidity of a fluid flowing in a flow tube, the device comprising:

-   -   a flow tube with a through-going opening for passage of a fluid        between an inlet and an outlet and comprising a turbidity        measurement section between a first section end and a second        section end;    -   a first transducer arranged for transmitting ultrasonic signals        through the fluid in the turbidity measurement section so as to        provide a first ultrasonic wave between the first and second        section ends;    -   a receiver transducer arranged for receiving ultrasonic signals        scattered on particles in the fluid flowing through the        turbidity measurement section; and    -   a control circuit connected to the first transducer and the        receiver transducer, the control circuit being arranged for        operating the first transducer and to generate a signal        indicative of the turbidity of the fluid in response to signals        received from the receiver transducer.

The use of ultrasonic waves for measurement of turbidity provides areliable and robust way of measuring turbidity without the problems withdisturbing coatings which causes problems in optical solutions.

The invention is based on the insight that particles in a fluid willscatter ultrasonic waves in the surrounding fluid.

According to the invention, the ultrasonic wave is generated in a flowtube between two boundaries, e.g. a wave based on applying to the firsttransducer an ultrasonic pure tone having a frequency f₀. A particlemoving along with fluid flowing in the flow tube will then pass theacoustic wave, and when passing a zone with high ultrasonic intensity,the particle will scatter ultrasonic waves with the frequency of thewave f₀ at a relatively high intensity. Correspondingly, the particlewill not scatter any ultrasonic waves when passing a zone with lowultrasonic intensity.

Further according to the invention, after demodulating with frequencyf₀, the receiver will exhibit a residual oscillation at a frequency,f_(s), depending on the velocity, v, with which the particles havetraveled along the wave. The residual oscillation frequency is given byf_(s)=(v/c)f₀, where c is the phase velocity of the ultrasound in thefluid. By filtering the demodulated signal, allowing only frequenciesaround f_(s) to pass, any background contributions from strayreflections etc. can be discriminated, since these, originating fromstationary sources, will contribute only to the DC component of thedemodulated signal.

Since the scattering probability of typical particles is expected to besmall, a high intensity of ultrasound is desired. This can be achievedby employing the flow tube as a resonator to the ultrasound, hencechoosing an ultrasound frequency so that the wave after a round tripback and forth in the measurement section will be reflected in-phasewith itself. The resulting intensity pattern will be a standing wave,having anti-nodes of high intensity and nodes of low intensity.

With little or no reflections, the standing wave will turn into (a)unidirectional traveling wave(s). Even in this case, however, the deviceaccording to the invention will still be able to measure the turbiditysince a particle moving with the flow scatters on the traveling wave.The frequency of the scattered ultrasound is downshifted in frequency ifthe flow and ultrasound propagation is in the same direction andupshifted in frequency if the flow and ultrasound propagation isopposite. This is known as the Doppler shift or Doppler effect. Thereceiver transducer will detect this frequency-shifted signal. Thisfrequency shift is comparable to the standing wave signal, but will notbenefit from the intensity enhancement of the resonator.

Thus it is recognized that even in the absence of a standing wavepattern, the ultrasound scattered by impurities travelling with thefluid and detected at the receiver, will exhibit the frequency f_(s)after demodulation with the carrier frequency f₀. The reason in thiscase is that the impurities are travelling with velocity v relative tothe ultrasonic source and hence experience a Doppler shifted ultrasonicfrequency, which is scattered into the receiver.

Back scattering Doppler turbidity sensors are known in the art. Theseemploy only a single transducer acting simultaneously as transmitter andreceiver. The demands to such a transducer are quite severe, since theoscillations from signal transmission must have died out at the time theecho reaches the transducer. Hence, the transducer must be efficientlymechanically damped, which in turn limits the coherence of theultrasonic wave and hence the accuracy of the sensor. Utilizing severaltransducers as is proposed in the present invention, allows foroptimizing these for their respective functions in the sensor andelimination of the above problem. Hence, the transmitter can be designedto oscillate at a high Q-value resonance resulting in high coherence,high intensity and high quality wave propagation pattern, and thededicated receiver can be designed to have high bandwidth and highsensitivity resulting in high frequency resolution of the scatteredultrasonic waves.

Also in the case of oblique angles between sound and fluid propagationthe oscillation frequency is modulated according to f_(s)=(v/c)f₀ cos φ,where φ is the angle between sound propagation direction and mean fluidvelocity.

Thus it is to be understood that according to the invention the firsttransducer may be arranged at said first section, creating a travellingwave propagating either parallel, antiparallel or at an oblique angledifferent from 90 degrees relative to the mean fluid velocity. Thereceiving transducer may be arranged perpendicular to the travellingwave propagation direction so that no reflections apart from the desiredscattered signal are detected. The electronic signal derived from thereceiving transducer is demodulated with the ultrasonic frequency f₀ andthe resulting signal is spectrally analysed to yield a quantityindicative of the turbidity level as the spectral density in a frequencyband around f_(s).

In an embodiment the first and second transducers are operating in atransient mode, where they emit wavepackets shorter than the length ofthe measurement section. Hence, the measurement section cannot performas a resonator but a transient standing wave pattern will still occur inthe region of the measurement section, where the wavepackets collide. Asa result the demodulated signal from the receiver will still exhibit thefrequency f_(s), but the technical complexity associated with trackingthe resonance of the measurement section is reduced. Noise suppressionand improvement of signal to noise ratio may be achieved bysynchronizing the receiver with the wavepacket generation using standardlock-in techniques.

The ultrasonic transducers may occasionally be operated in a fashionappropriate for a time-of-flight or Doppler flow meter. It is reasonableto assume that the particles move with the same velocity as the fluid,hence the flow measurement readily yields the value of v, using twotransducers. Consequently, knowledge of the fluid velocity implies thatthe expected values of f_(s) is known, meaning that an adaptive digitalfilter can be employed to select a band around this frequency therebyrejecting background noise. Such filtering techniques are well known inthe art of digital signal processing.

In practice the speed of sound, c, varies appreciably with thetemperature of the fluid. The ultrasonic flow meter operation mode ofthe turbidity sensor may accordingly also be utilised to provide ameasurement of the fluid temperature as well. This becomes apparent,since the essence of the time-of-flight flow measurement is the timingof two ultrasonic wave packets travelling between two transducers co-and counter-propagating to the fluid flow respectively. The length ofthe measurement section divided by the average travelling time of thetwo wave packets results in the speed of sound in the fluid.

Alternatively, a separate, dedicated temperature sensor may be includedin the turbidity sensor in order to determine the temperature dependentvalue of c by means of a lookup table.

It is to be understood that it is still possible to provide a measure ofturbidity without a measure of flow rate of the fluid, however,resulting in reduced reliability. Alternatively, a more complicatedsignal processing will in such case be necessary in order to provide areliable measure of turbidity.

Also it should be understood that c is temperature dependent. Althoughit is still possible to provide a measure of turbidity based ontemperature estimates, a more reliable turbidity measurement may beobtained in those cases of in-situ measurements of the fluidtemperature, e.g. by means of a temperature sensor integrated with thedevice.

In the long wavelength regime, the power of ultrasonic waves scatteredfrom a particle is proportional to the 4^(th) power of the ultrasonicwave frequency, and proportional to the 2^(nd) power of the volume ofthe particle. For example, this means that the ultrasonic response fromparticles with a size of 5-10 μm, e.g. larger colloides, grains of sand,particles of clay, organic material etc. can be observed using anultrasonic standing wave at a frequency f₀ being in the MHz range, e.g.5-10 MHz. Quantifying the amount of such particles provides a measure ofturbidity of a fluid, e.g. water in a drinking water utility network,which is a useful measure of general cleanness of the fluid. Thetechnique may even have a sensitivity allowing for the detection ofmicrobes, amoebae and bacteria in sufficiently high concentrations.

Generation and measurements of ultrasonic waves in the MHz frequencyrange is possible by normal low cost components, and thus similartechnology as known in ultrasonic flow meters. Thus, a turbiditymeasurement device can easily be combined and integrated with existingultrasonic flow meters, e.g. as known in ultrasonic consumption meters.Hereby, the turbidity measurement facility can be provided by a singlecomponent without the need for installing a separate device to monitorturbidity of a fluid in a utility network. Even further, it isadvantageous to combine the turbidity device with an ultrasonic flowmeter, since hereby the flow rate can be provided for use in theturbidity calculation, as explained above. By integrating the turbidityfunctionality into a consumption meter, the wireless communicationnetwork of such consumption meters can be used for turbidity data aswell, thereby allowing collection of turbidity data from the location ofa large number of consumers, e.g. for further processing which can helpto determine the location of a source of contamination in the pipingnetwork.

It is to be understood that the first and the second ‘section end’indicate ends of the turbidity measurement section, i.e. ends of thesection wherein the ultrasonic wave extends along the fluid direction inthe flow tube.

By ‘control circuit’ is understood the necessary electronic circuitsadapted to control the function of the ultrasound transducer(s), i.e. togenerate electric signals to drive the first transducer, and to receiveelectric signals from the receiver transducer.

In the following, preferred features and embodiments will be described.

The first transducer may be arranged at said first section end, andwherein a reflecting element is arranged at the second section end, e.g.to constitute the second section end, for reflecting the ultrasonicsignals. Especially, the first transducer may be arranged at a centralpart of the flow tube cross sectional area and facing an ultrasonicreflecting element also arranged at a central part of the flow tubecross sectional area, e.g. such as at a distance of 5-15 cm from thefirst transducer, thus allowing the ultrasonic wave become a standingwave between the first transducer and the reflecting element. The firsttransducer and the reflecting element preferably occupy only a limitedfraction of the flow tube cross section area, so as to allow the fluidto flow around these parts without creating any significant disturbanceor turbulence in the fluid.

The device may comprise a second transducer arranged at the secondsection end, so as to provide a second ultrasonic wave between thesecond and first section ends, and wherein the control circuit isarranged for operating both of the first and second transducers.Preferably, the control circuit is capable of driving the first andsecond transducers to generate said first and second ultrasonic wavessimultaneously. The first and second ultrasonic waves may have similarfrequencies, thus the control circuit may be arranged to applyelectrical signals, e.g. pure tone signals, with the same frequency e.g.within 1-100 MHz, or more specifically within 2-20 MHz, such as 5-10MHz. Especially, the same electrical signal may be applied to both ofthe first and second transducers. Alternatively, the first and secondultrasonic waves may have different frequencies. E.g. a frequency of thefirst ultrasonic wave may be a rational number p/q times the secondfrequency of the second wave. This may be advantageous, since in orderto provide a high acoustic output, the transducers used may be drivennear or at their mechanical resonance frequency, and thus using the sametype of transducer for the first and second transducers, driving onetransducer at an odd harmonic of its mechanical resonance frequency willstill provide a high acoustic output. Further, a frequency of the firstultrasonic wave and a second frequency of the second ultrasonic wave maydiffer by 0.1% to 10%. The first and second waves are preferablyspatially overlapping, e.g. with the first and second transducersarranged at the respective first and second section ends.

In this context, the invention further provides an ultrasonic flow metercomprising first and second ultrasonic transducers arranged to generaterespective first and second ultrasonic waves in a flow tube, wherein thea first frequency of the first ultrasonic wave is different from asecond frequency of the second ultrasonic wave. The flow meter comprisesa control circuit connected to operate the first and second transducers,and being arranged to generate an output indicative of a flow rate ofthe fluid in the flow tube, preferably in response to sensing anultrasonic response from scattering on particles of the fluid flowingalong the first and second ultrasonic waves. Reference is made to theforegoing paragraph with respect to embodiments of such flow meter withrespect to how the frequencies of the first and second ultrasonic wavesmay be different.

The control circuit may be arranged to generate the signal indicative ofthe turbidity of the fluid in response to signals received from thereceiver transducer and a flow rate of the fluid. Especially, as alreadymentioned, in case of an ultrasonic wave with frequency f₀, the expectedsignal frequency f₀±f_(s) of ultrasonic waves received by the receivertransducer is: f_(s)=(v/c)f₀ cos φ. Here φ is the angle between soundpropagation direction and mean fluid velocity direction, v is thevelocity of the particles responsible for the turbidity and c is thespeed of the ultrasonic wave in the fluid. This is preferably utilizedin the processing in the control circuit, e.g. to demodulate thereceived signal from the receiver transducer, and to subsequently filterthe resulting signal, so as to observe the expected frequency f_(s) inorder to suppress background noise, and thus provide a more reliablemeasure of turbidity. Especially, the control circuit may be arranged togenerate the signal indicative of the turbidity of the fluid in responseto an output from a predetermined algorithm in response to both of: ameasured flow rate of the fluid in the flow tube, and a frequency of theultrasonic standing wave. E.g. said predetermined algorithm may involvecalculating a level of the ultrasonic signals received by the receivertransducer at one or more frequency components selected in response toboth of: the measured flow rate of the fluid in the flow tube, and thefrequency of the ultrasonic standing wave.

In an embodiment of the invention the first ultrasonic wave is astanding wave.

In an alternative embodiment of the invention the first ultrasonic waveis a travelling wave.

In an embodiment of the invention the first and the second ultrasonicwaves are standing waves.

In an alternative embodiment of the invention the first and the secondultrasonic waves are travelling waves.

The application of standing waves provides areas of high acousticintensity at the antinodes of the wave, and thus high scatteringintensity.

Compared to the application of standing waves, the application oftravelling waves, i.e. non-standing waves, provides higherfreedom-of-design of the device.

With an embodiment of the invention the first and second ultrasonicwaves are transient waves of similar frequency in the form of wavepackets, which are shorter than the distance between the first sectionend and the second section end, so as to form a transient standing wavein at least part of the turbidity measurement section.

The device may comprise flow measurement means so as to provide a flowrate to be used by the control circuit to calculate a more reliablemeasure of turbidity, as explained above. The flow rate may be providedby an external device, or it may be measured by an integral flow meterof the device itself. Especially, the flow measurement means maycomprise the first transducer, and preferably also a second transducer,and a control circuit operating the first and second transducersaccording to a time-of-flight or Doppler principle. Hereby, a combinedflow meter and turbidity meter can be provided utilizing the sameultrasound transducer and control circuit. Especially, the controlcircuit may be arranged to operate the first transducer in a first and asecond operation time intervals, wherein the first and second operationtime intervals are not overlapping, wherein the control circuit isarranged to operate the first transducer for measuring the turbidity ofthe fluid flowing in the flow tube during the first operation timeinterval, and wherein the control circuit is arranged to operate thefirst transducer for measuring the flow rate of the fluid flowing in theflow tube during the second operation time interval. This allowsreliable flow rate and turbidity measurements without interferencebetween the ultrasonic signals involved in the two types ofmeasurements, even though the same transducer, or the same set oftransducers, are involved in both types of measurements.

The control circuit may be arranged to operate the first transducer at afirst frequency for measuring the turbidity, and being arranged tooperate the first transducer at a second frequency for measuring theflow rate, such as the first frequency being higher than the secondfrequency, such as the first frequency being the odd harmonic, like thethird harmonic of the first frequency. This frequency difference allowsflow rate to be measured at a reasonably low frequency, while turbiditymeasurements can be provided at a higher frequency to increasesensitivity to detect small particles in the fluid. Especially, thefirst frequency may be above 1 MHz, such as above 10 MHz, and whereinthe second frequency is below 5 MHz, such as below 2 MHz.

The control circuit may be arranged to calculate a level of ultrasonicsignals received by the receiver transducer, and to generate the signalindicative of the turbidity of the fluid accordingly, without anyknowledge about fluid flow rate in the flow tube. For some applications,the turbidity precision that can be obtained in this way may besufficient, however a more complicated data processing may be requiredcompared to methods utilizing fluid rate—either provided from anexternal device, or by the integration of the turbidity device with aflow meter, as already described.

The control circuit may be arranged to generate the signal indicative ofthe turbidity of the fluid in response to an average of measured valuesover a period of time, hereby allowing only a limited amount of measuredturbidity values to be communicated.

The device may comprise temperature measurement means arranged tomeasure a temperature of the fluid. Especially, said temperaturemeasurement means may comprise the first transducer, and preferably alsoa second transducer. Thus, in some embodiments, the first transducersmay be used for turbidity measurements, flow rate measurements, as wellas temperature measurements.

The device may comprise a first ultrasonic reflector arranged to guideultrasonic signals from the first transducer in a direction opposite theflow direction in the turbidity measurement section. In embodimentscomprising a second transducer, a second ultrasonic reflector may bearranged to guide ultrasonic signals from the second transducer in adirection of the fluid flowing. Especially, such first and secondultrasonic reflectors may constitute the first and second end sectionsbetween which the ultrasonic wave extends. Such reflectors allow thefirst (and possibly second) transducer(s) to be arranged away from acentral part of the fluid flow, e.g. with the(se) transducer(s) to bepositioned at or near the wall of the flow tube.

Also according to the invention the receiver may have a receivingsurface which is parallel to a direction of the first ultrasonic wave.

The device may comprise an acoustic lens or an aperture arranged inrelation to the receiver transducer, so as to limit area volume of theturbidity measurement section from which ultrasonic signals can reachthe receiver transducer and so discriminate against unwanted backgroundfrom stray scattering of ultrasound.

The receiver transducer may be arranged in an opening in a wall of theflow tube, such as the receiver transducer being arranged with itsreceiver surface retracted behind a surface covering said opening. Thus,the receiver transducer may be retracted in a well, thus serving toreduce unwanted ultrasonic reflections from the flow tube in reachingthe receiver transducer, thereby increasing the turbidity measurementprecision. The receiver transducer is preferably arranged between thefirst and second section ends, e.g. centrally between said section ends,such as with its receiver surface forming a plane containing the flowdirection of the fluid flowing in the turbidity measurement section.

Such arrangements of the receiver transducer allow for efficientreception of scattered signal at high signal-to-noise ratio.

The receiver transducer may comprise a plurality of separate transducersarranged at respective positions along the turbidity measurementsection. Such array of separate receiver transducers spatiallydistributed along the ultrasonic standing wave may provide an improvedresponse signal for further processing compared to a single receivedtransducer.

The first transducer may comprise a piezoelectric transducer.Especially, the piezoelectric transducer may exhibit a mechanicalresonance frequency coinciding with a frequency of a drive signalapplied to the piezoelectric transducer by the control circuit. Thisallows the transducer to provide a high acoustic output, and thusprovide an ultrasonic standing wave with a high intensity, therebyallowing scattered signals from particles in the fluid to allow a robustturbidity measurement. Especially, the control circuit may be arrangedto operate the first transducer at said mechanical resonance frequencyfor measurement of flow rate, and wherein the control circuit isarranged to operate the first transducer at a higher frequency formeasurement of turbidity, and wherein said higher frequency is selectedto coincide with an odd harmonic of said mechanical resonance frequency.Hereby, the same transducer can be used to provide a high acousticoutput at the two different frequencies for turbidity and flowmeasurements, respectively.

The first transducer may be arranged in a central part of a crosssection of the flow tube, and e.g. constitute the first section end.

The receiver transducer may comprise a piezoelectric transducer.

The device may comprise a liner formed by a material comprising apolymer for covering at least part of a surface of a measurement tube inthe turbidity measurement section.

In some embodiments, the device may be integrated or combined with anultrasonic flow meter which may be or may be part of a chargingconsumption meter or utility meter, e.g. a water meter for cold and/orhot water, a heat meter, a cooling meter, or a gas meter, where theconsumption meter is arranged for measuring consumption data of asupplied utility used as a basis for billing. The consumption meter maybe used in connection with district heating, district cooling and/ordistributed water supply. The consumption meter may be a legal meter,i.e. a meter which is subdued to regulatory demands. Such regulatorydemands may be demands to the precision of the measurements.Advantageously, the flow meter can be used as a water meter, thusallowing measurement of the amount of particles (i.e. turbidity) in thesupplied water.

Especially, in case the device comprises an ultrasonic flow ratecapability, the control circuit preferably comprises a measurementcircuit to allow measurement of fluid flow according to known principlesof ultrasonic transit time. Especially, the measurement circuit may bearranged on one single printed circuit board (PCB) capable of generatingas output a pulse train indicative of the measured fluid flow rate. Onesingle processor may be used to handle measurement of both flow rate andturbidity, however separate processors may as well be provided forcalculation of flow rate and turbidity.

The device may comprise a communication module connected to the controlcircuit and arranged for communicating the signal indicative of theturbidity of the fluid, e.g. turbidity data may be communicated as awireless radio frequency signal.

A specific embodiment of the first aspect provides a device, wherein thefirst transducer is arranged at the first section end, and wherein asecond transducer is arranged at the second section end, so as toprovide a second ultrasonic standing wave between the second and firstboundaries, and wherein the control circuit is arranged for operatingboth of the first and the second transducers, wherein the controlcircuit is arranged to generate the signal indicative of the turbidityof the fluid in response to signals received from the receivertransducer and a flow rate of the fluid, further comprising flowmeasurement means arranged to measure the flow rate of the fluid flowingin the flow tube, wherein said flow measurement means comprises thefirst and second transducers, wherein the control circuit is arranged tooperate the first and second transducer in a first and second operationtime intervals, wherein the first and second operation time intervalsare non-overlapping, wherein the control circuit operates the first andsecond transducers for measuring the turbidity of the fluid flowing inthe flow tube during the first operation time interval, and wherein thecontrol circuit operates the first and second transducers for measuringthe flow rate of the fluid flowing in the flow tube during the secondoperation time interval.

Such device is advantageous, since it can use a set of ultrasonictransducers known from existing ultrasonic flow meters, e.g. ultrasonicconsumption meters, thus with limited modifications it is possible toprovide a device capable of providing a measure of turbidity of thefluid. Especially, it may be considered to be advantageous to use ahigher ultrasonic frequency, e.g. higher than 5 MHz, e.g. 10 MHz, forthe turbidity measurement, while a lower ultrasonic frequency can beused, e.g. 1-2 MHz for the flow measurements. The device may furthercomprise a wireless communication module arranged to transmit both of:data indicative of the measured turbidity, and data indicative of themeasured flow rate and/or a consumed amount, so as to allow remotereading of turbidity over the same communication channel used for remotereading of utility data.

In the second aspect, the invention provides an ultrasonic consumptionmeter comprising a device according to the first aspect, such as theultrasonic consumption meter being a water meter, a gas meter, a heatmeter, or a cooling meter.

In a third aspect the invention provides a system for monitoringturbidity of fluid in a utility network, the system comprising

-   -   a plurality of devices or ultrasonic consumption meters        according to the first or second aspects, wherein each of the        plurality of devices are arranged to transmit signals indicative        of the turbidity of the fluid, and    -   a communication system arranged to mediate said signals        indicative of the turbidity of the fluid from the plurality of        devices or ultrasonic consumption meters. Optionally, the system        may comprise a processor system arranged to analyze said signals        indicative of the turbidity of the fluid.

In a fourth aspect, the invention provides a method of measuringturbidity of a fluid flowing in a turbidity measurement section of aflow tube, the method comprising:

-   -   transmitting ultrasonic signals from a first transducer to        generate an ultrasound wave between a first section end and a        second section end,    -   receiving, by means of a receiver transducer, ultrasonic signals        scattered on particles in the fluid, and    -   generating a signal indicative of the turbidity of the fluid in        response to signals received from the receiver transducer.

The same advantages mentioned for the first aspect apply as well for thesecond, third, and fourth aspects. In general, the various aspects ofthe invention may be combined and coupled in any way possible within thescope of the invention. These and other aspects, features and/oradvantages of the invention will be apparent from and elucidated withreference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described, by way of example only,with reference to the drawings, in which

FIG. 1 illustrates a sketch of an embodiment with two transducersgenerating an ultrasonic standing wave between them, and the receivertransducer arranged at the wall of the flow tube,

FIG. 2 illustrates a sketch of an embodiment similar to that of FIG. 1but with reflectors serving to direct ultrasonic waves from the twotransducers,

FIG. 3 illustrates an embodiment similar to FIG. 1 but with a pluralityof receiver transducers distributed along the flow tube,

FIG. 4 illustrates a sketch of an embodiment with one transducergenerating an ultrasonic travelling wave, and the receiver transducerarranged at the wall of the flow tube,

FIG. 5 illustrates a system embodiment with a plurality of water meterswith turbidity measurement facilities connected to a utility net andcommunicating turbidity data to a central facility for processing,

FIG. 6 shows the variation of the residual frequency f_(s) with the flowvelocity v,

FIG. 7 shows the variation of the intensity of the frequency shiftedsignal vs. the turbidity level, and

FIG. 8 illustrates steps of a method embodiment.

DESCRIPTION OF EMBODIMENTS

FIG. 1 illustrates a sketch of a turbidity measurement device embodimentwith a flow tube with walls W, where two transducers T1, T2 are arrangedin the flow tube and serve as end sections for the turbidity measurementsection TMS between which two ultrasonic standing waves are generated.The dark circles indicate particles flowing along the fluid in the flowtube with flow rate v. A receiver transducer R1 is arranged at the wallW of the flow tube at a central position between the two transducers T1,T2. For one particle an ultrasonic response to the ultrasonic standingwave is indicated with a dashed arrow towards the receiver transducer R1which then captures the ultrasonic response from the particle. With anultrasonic standing wave with frequency f₀, the expected frequency f_(s)of high intensity scattering of a particle in a fluid with flow rate vis: f_(s)=(v/c)f₀, where c is the speed of the ultrasonic wave in thefluid. Thus, a high intensity response from particle scattering of theultrasonic standing wave can then be expected with a period time ofP=1/f_(s) at the receiver transducer, as indicated in the responseversus time t to the upper right corner of FIG. 1.

A control circuit CC comprises an electric generator that applieselectric drive signals to the transducers T1, T2 at the single frequencyf₀, receives the response from the receiver transducer R1 and generatesin response a signal indicative of turbidity TB. As indicated, thesignal from the generator may be applied to a multiplier together withthe response from the receiver transducer R1, thus demodulating thereceived signal. Further, the control circuit CC preferably applies afiltering, e.g. involving Fast Fourier Transform finite impulse responseor infinite impulse response digital filters, so as to explore theactual high intensity response from particles at the expectedperiodicity P. The resulting signal can then be quantified so as toprovide a measure of particle density in the fluid, i.e. a measure ofturbidity.

It is to be understood that the same two transducers T1, T2 can be usedas well for ultrasonic flow rate measurement, such as known in the art,and thus preferably the flow rate v can be measured with the device aswell, thus delivering the flow rate v to the control circuit, therebyallowing the above described calculation of f_(s).

Not shown, the receiver transducer R1 may be retracted from the flowtube wall W, so as to receive only ultrasonic response from a limitedportion of the flow tube, rather than all response includingreflections.

FIG. 2 shows an embodiment similar to FIG. 1 except for the position ofthe two transducers T1, T1, since here section ends of the turbiditymeasurement section TMS are constituted by respective ultrasonicreflectors RF1, RF2, e.g. polymeric, composite, or metallic reflectors.Thus, the transducers T1, T2 are positioned out of the fluid flow, alongthe wall W of the flow tube, and their ultrasound signals are thendirected in the flow direction, such that the ultrasonic standing wavesbetween the reflectors is along the flow direction.

FIG. 3 shows an embodiment similar to FIG. 1 except for the use of 6separate receiver transducers R1-R6 arranged along the flow tube wall.The responses from these receiver transducers R1-R6 are then combined inthe processing of the control circuit to result in one single measure ofturbidity TB.

FIG. 4 illustrates an alternative embodiment of the invention. Comparedto the embodiment of the invention of FIG. 1, this embodiment comprisesonly one transducer T1, and the ultrasonic wave is a travelling wave,i.e. a non-standing wave, as it is only scarcely reflected at thesection end E2, or not at all.

The intensity response from particle scattering will not be enhanced bythe cavity build up enhancement factor, but it will still exist owing tothe Doppler effect. The expected frequency remains f_(s), as describedabove.

FIG. 5 shows a system embodiment. Two groups G1, G2 of water meters W_Mare connected to measure consumed water at respective consumers on awater utility network U_N. The water meters W_M are arranged to measureturbidity according to the present invention, preferably using one ortwo ultrasonic transducers which are also involved in flow ratemeasurement for generating a measure of consumed water. Consumed waterdata and turbidity data are transmitted wirelessly by the water metersW_M to a central communication module which extracts the turbidity dataTB_D which are then applied to a data processing DP for furtheranalysis. E.g. in case turbidity is generally higher in group G1 thangroup G2, it may be used as an indicator that a leak in the pipingsystem between the positions of the two groups G1, G2 of water metersW_M allows soil or other contamination in the water utility network U_N,and thus helps in finding such broken pipe. Otherwise, the turbiditydata TB_D may be used to generally monitor water quality delivered tothe consumers.

FIG. 6 shows the variation of the residual frequency f_(s) with the flowvelocity v.

A device according to the invention as sketched with FIG. 1 wasconnected to a flow system having a constant turbidity. The transducerwas driven with a constant frequency in the range 5-15 MHz and anexternal time-of-flight flow meter was measuring the flow rate. Thesignal from a receiver transducer, placed in the turbidity measurementsection, was collected and its frequency shift f_(s) analyzed. The flowvelocity based on flow measurement is shown on the x-axis. The y-axisrepresent the frequency shift where the carrier frequency has been usedas units. As can be seen from the figure, the frequency shift varieslinearly with flow velocity according to the principle of the residualoscillation frequency of the invention.

FIG. 7 shows the variation of the intensity of the frequency shiftedsignal vs. the turbidity level.

The setup described in FIG. 6 was employed with a constant flowvelocity, i.e. the frequency shift is constant. A series of measurementwith varying turbidity (based on a 4000 NTU polystyrene standard) of thefluid flow was conducted. The intensity of the frequency shifted signalis analyzed and plotted as a function of the turbidity. A clearmonotonic correspondence is seen between the turbidity and receiverresponse, even over the broad operational range of the sensor.

FIG. 8 shows steps of a method embodiment for measurement of turbidityof a fluid flowing in a flow tube. First, an ultrasound signal istransmitted T_US_F1 at a first frequency from a first transducer troughthe fluid, to generate a first ultrasonic standing wave in the flowtube. A response is received R_US_1 by means of a receiver transducercapturing ultrasonic signals scattered on particles in the fluid, andgenerating G_TB a signal indicative of the turbidity of the fluid inresponse to signals received from the receiver transducer. Further,another ultrasonic signal is transmitted T_US_F2 from the transducer ata second frequency which is lower than the first frequency. A responsethereto is received R_US_2 at a second transducer, and in response asignal indicative of flow rate of fluid flowing in the flow tube isgenerated G_FR accordingly. Preferably, this flow rate is used in thegeneration of the signal indicative of the turbidity.

To sum up, the invention provides a turbidity measurement device formeasuring turbidity of a fluid flowing in a flow tube. A firsttransducer transmits ultrasonic signals through the fluid in theturbidity measurement section so as to provide a first ultrasonic wavebetween the first and second section ends. A receiver transducerreceives the ultrasonic scattered response from particles in the fluidflowing through the turbidity measurement section. A control circuitoperates the transducers and generates a signal indicative of theturbidity of the fluid in response to signals received from the receivertransducer. Preferably, the device may comprise a second transducer forgenerating a second ultrasonic wave with the same frequency, and furtherthe two transducer may be used to generate a measure of flow rate bymeans of known ultrasonic techniques. This flow rate may be used in thecalculation of a measure of turbidity. Both turbidity facilities andflow rate facilities may be integrated in a consumption meter, such as aheat meter or a water meter.

Although the present invention has been described in connection with thespecified embodiments, it should not be construed as being in any waylimited to the presented examples. The invention can be implemented byany suitable means; and the scope of the present invention is to beinterpreted in the light of the accompanying claim set. Any referencesigns in the claims should not be construed as limiting the scope.

The invention claimed is:
 1. A device arranged to measure turbidity of afluid flowing in a flow tube, the device comprising: a flow tube havinga through-going opening for passage of a fluid between an inlet and anoutlet and a turbidity measurement section between a first section endand a second section end, a first transducer arranged to transmitultrasonic signals having a frequency f₀ through the fluid in theturbidity measurement section so as to provide a first ultrasonic wavebetween the first and second section ends, a second transducer arrangedto transmit ultrasonic signals through the fluid in the turbiditymeasurement section, so as to provide a second ultrasonic wave betweenthe second and first section ends, a receiver transducer arranged forreceiving ultrasonic signals scattered on particles in the fluid flowingthrough the turbidity measurement section, wherein the receivertransducer has a receiving surface which is parallel to a propagationdirection of the first ultrasonic wave, and a control circuit connectedto the first transducer, the second transducer, and the receivertransducer, the control circuit being arranged to operate the firsttransducer and the second transducer, demodulate signals received fromthe receiver transducer such that the receiver transducer exhibits aresidual oscillation at a frequency f_(s), and to generate a signalindicative of the turbidity of the fluid in response to signals receivedfrom the receiver transducer based, at least in part, on the frequencyf_(s), and a flow rate of the fluid.
 2. The device according to claim 1,wherein the first transducer is arranged at said first section end, andwherein a reflecting element is arranged at the second section end forreflecting the ultrasonic signals.
 3. The device according to claim 1,wherein the first and second ultrasonic waves have similar frequencies.4. The device according to claim 1, wherein a frequency of the firstultrasonic wave is a rational number p/q times a frequency of the secondultrasonic wave, or wherein a frequency of the first ultrasonic wave anda frequency of the second ultrasonic wave differ by 0.1% to 10%.
 5. Thedevice according to claim 1, wherein the first and the second ultrasonicwaves are standing waves.
 6. The device according to claim 1, whereinthe first and second ultrasonic waves are transient waves of similarfrequency in the form of wave packets, which are shorter than thedistance between the first section end and the second section end, so asto form a transient standing wave in at least part of the turbiditymeasurement section.
 7. The device according to claim 1, wherein thefirst ultrasonic wave is a standing wave.
 8. The device according toclaim 1, comprising flow measurement means, wherein said flowmeasurement means comprises the first transducer.
 9. The deviceaccording to claim 8, wherein the control circuit is arranged to operatethe first transducer in a first and a second operation time interval,wherein the first and second operation time intervals are notoverlapping, wherein the control circuit is arranged to operate thefirst transducer for measuring the turbidity of the fluid flowing in theflow tube during the first operation time interval, and wherein thecontrol circuit is arranged to operate the first transducer formeasuring the flow rate of the fluid flowing in the flow tube during thesecond operation time interval.
 10. The device according to claim 8,wherein the control circuit is arranged to operate the first transducerat a first frequency for measuring the turbidity, and is furtherarranged to operate the first transducer at a second frequency formeasuring the flow rate.
 11. The device according to claim 1, comprisingtemperature measurement means, wherein said temperature measurementmeans comprises the first transducer.
 12. The device according to claim1, comprising a first ultrasonic reflector arranged to guide ultrasonicsignals from the first transducer in a direction of the fluid flowing inthe turbidity measurement section.
 13. The device according to claim 1,wherein the receiver transducer is arranged in an opening in a wall ofthe flow tube.
 14. The device according to claim 1, comprising anacoustic lens or an aperture arranged in relation to the receivertransducer, so as to limit a volume of the turbidity measurement sectionfrom which ultrasonic signals can reach the receiver transducer.
 15. Anultrasonic consumption meter comprising a device according to claim 1.16. A system for monitoring turbidity of fluid in a utility network, thesystem comprising: a plurality of devices according to claim 1, whereineach of the plurality of devices is arranged to transmit signalsindicative of the turbidity of the fluid, a communication systemarranged to mediate said signals indicative of the turbidity of thefluid from the plurality of devices, and a processor system arranged toanalyze said signals indicative of the turbidity of the fluid.
 17. Thesystem for monitoring turbidity of fluid in a utility network accordingto claim 16, wherein at least one of the plurality of devices isarranged to be a leak indicator in the event the turbidity of the fluidassociated with the at least one of the plurality of devices is higherthan the turbidity of the fluid associated with a different at least oneof the plurality of devices.
 18. The device according to claim 1,wherein: the through-going opening of the flow tube defines a flow pathfor passage of the fluid; the first transducer is arranged to transmitultrasonic signals along a path through the fluid in the turbiditymeasurement section; and the receiver transducer is positioned outsideof the flow path of the fluid and outside of the path of the transmittedultrasound signals.
 19. A method of measuring turbidity of a fluidflowing in a turbidity measurement section of a flow tube, the methodcomprising: transmitting ultrasonic signals having a frequency f₀ from afirst transducer to generate an ultrasound wave between a first sectionend and a second section end, transmitting ultrasonic signals from asecond transducer to generate a second ultrasound wave between the firstsection end and the second section, receiving, by means of a receivertransducer having a receiving surface which is parallel to a propagationdirection of the first ultrasonic wave, ultrasonic signals scattered onparticles in the fluid, demodulating signals received from the receivertransducer with the frequency f₀ such that the receiver transducerexhibits a residual oscillation at a frequency f_(s), and generating asignal indicative of the turbidity of the fluid in response to signalsreceived from the receiver transducer based, at least in part, on thefrequency f_(s), and a flow rate of the fluid.